1. Field of the Invention
The present invention relates to a mixing head for a reaction injection molding machine that mixes a plurality of reactive polymeric components for subsequent injection into a mold. The head is of the type used primarily, for example, in mixing urethane or isocyanurate polymer compositions.
Reaction injection molding, also called liquid injection molding, is a technique for combining reactive liquid polymeric components and injecting them into a mold where they rigidify to form a finished product. The component combination is achieved by directing streams of two or more reactive polymeric liquid components, each under high pressure, to cause their impingement at a common point in a mixing chamber of the mixing head. The resultant component impact creates a homogeneous mass of material in the mixing chamber which is generally then injected under pressure into a closed mold to which the mixing head is connected, or the mix may simply be dispensed into an open mold.
The reaction injection molding technique has advantages over conventional plastic foam open molding techniques in that a completely closed system is maintained for the components from withdrawal from their respective supply tanks to their injection in admixed condition into the closed mold. This totally enclosed injection molding technique permits formation of more intricate, delicate products, having narrow or thin sections that could not oridinarily be formed by conventional low pressure or open mold techniques. It also permits use of components of greater reactivity than is practical in the low pressure open mold techniques.
However, mixing heads used to combine reactive polymeric components and to inject the admixture of these components into a closed mold have exhibited certain problems.
2. Description of the Prior Art
Various forms of mixing heads for reaction injection molding machines are presently known. One typical form, disclosed, for example, in U.S. Pat. Nos. 3,706,515 (Keuerleber et al.) and 3,102,004 (Grintz); and in the British Patent Specifications 1,407,616 (Hodgson) and 1,401,752 (Krauss-Maffei Aktiengellschaft), includes a body that has a bore which defines a mixing chamber. A plurality of nozzle orifices open into the mixing chamber for conducting reactive polymeric components thereto. The orifices are ordinarily directed at a common point in the mixing chamber to effect violent impingement of each component with all others to accordingly mix the components together into a homogeneous fluid mass. Flow of all components through all nozzle orifices is simultaneously controlled by a plunger mounted for axially reciprocal movement in the mixing chamber. When the plunger is retracted into the head, the orifices in the mixing chamber are opened, permitting polymeric components to issue therefrom in the form of high velocity impinging streams.
Such mixing heads also include a plurality of return ducts each of which opens into the mixing chamber at a location axially displaced from its respective component nozzle orifice. The plunger is formed with a plurality of by-pass channels which respectively communicate, when the plunger is in a recycle position, between an adjacent nozzle orifice and return duct of each component to establish a closed loop leading back to the component supply to provide recirculation. The plunger recycle position corresponds to maximum extension in the head, that is, the extreme of reciprocal plunger travel opposite to the retracted, nozzle orifice-opening, position.
Ordinarily in reaction injection molding processes, the components are pressurized to levels which often exceed 2000 to 2400 psi. Because of this, and because of reactivity of the several components if there is leakage around the plunger, the tolerances between moving parts of the mixing head, such as those between the plunger and mixing head, must be maintained extremely close. For example, tolerances of .+-.0.0001 inch for the plunger and bore diameters are not uncommon. In the past, it has been found that the need for such extremely small tolerances has shortened the operational life of mixing heads because the plunger has a tendency to bind or seize in the mixing head body.
Further, the plunger is usually driven by a coaxially mounted hydraulic piston and, while not fully appreciated heretofore, because of the extremely small tolerances between the plunger and mixing head body, slight colinear misalignment between the actuating piston and the plunger may produce a tendency of the plunger to bind in the mixing head body. This also contributes to shortened operational life.
Still another problem encountered with prior art reaction injection molding machine mixing heads relates to control of the volume of polymeric component entering the mixing chamber through each nozzle orifice when these are opened. Typically, prior art apparatus employ a needle-type restrictor valve to control the flow through each nozzle orifice and, hence, the amount of polymeric component which issues from it when it is open. However, again because of the high pressures at which such apparatus operate, these restrictor valves tended to leak if adjustment is attempted while the component is under injection pressure.
The mixing head for the reaction injection molding machine of the present invention has been designed to minimize the problems noted above.